Various materials have been employed as release layers, i.e., layers capable of releasing, on demand, layers or surfaces in contact with or in close proximity thereto. A first exemplary class of agents includes surface functionalization to reduce adhesion. These compounds are predominantly self-assembled monolayers (“SAM”)—primarily silanes—that impart hydrophobicity to surfaces. Their low surface energy and hydrophobicity/hydrophilicity mismatch are intended to promote release of materials from coated surfaces. Such materials generally function only to minimize adhesion and typically do not have controllable surface energies.
A second exemplary class of materials includes specialized polymers having chemical groups with arrangements reconfigurable upon application of heat, thus enabling hydrophobicity tuning. These materials are exemplified by N-isopropyl acrylamide (“NIPAAM”), which is a polymer having a controllable surface energy. The material has a transition point at approximately 36° C., above which the polymer chains dehydrate and collapse upon each other, forming a hydrophobic layer. Below the critical temperature, the polymer chains are hydrated and hydrophilic. In the past, the polymer has been grafted on various surfaces, including glass, and has been used for temperature-controlled release of cultured cells. Above 36° C. (the nominal temperature for culturing cells is 37° C.), cells adhere to NIPAAM-coated surfaces, but at room temperature the cells may be lifted off from the polymer.
A third exemplary class of materials includes precursor additives that are mixed with a curable polymer material to impart low surface-binding energy to the bulk compound. For example, ultraviolet—(“UV”) curable additives have been employed in this manner. The precursor typically includes a polymerizing moiety attached to a hydrophobic group. The precursor is integrated with the bulk material through UV curing, which forms the cast while maintaining a low surface energy to reduce adhesion of a molded part.
However, none of these materials satisfy all of the following criteria for robust micro/nanolithography: (i) cost effectiveness; (ii) ease of processing and/or coating onto substrates; (iii) compatibility with micro/nanolithography processing; and (iv) tunability of surface adhesion.
For example, with SAM chemistries, surfaces are typically permanently rendered minimally adhesive. The functional group imparts low binding energy to the surface, which is generally not compatible with thin coatings that are typically needed for multi-layer micro/nanostructuring. With thin, uniform coatings, the material is usually spun onto the surface, and therefore an additional adhesive layer is needed for effective distribution under the centripetal spinning force.
The NIPAAM polymer enables thermal control of surface adhesion, but is difficult to process and implement. In addition, thin coatings that do not obstruct or distort the underlying micro/nanofeatures of a cast have not been achieved with NIPAAM and may not be possible since the nominal thickness of the NIPAAM polymer has been estimated to be at least 70-80 nm. NIPAAM grafting is generally also time intensive, typically requiring special solvents and procedures, including a nitrogen environment for successful polymerization. NIPAAM's low transition temperature also typically precludes robust micro/nanoprocessing, where temperatures greater than 36° C. are often used to process lithographic materials.
UV-curable additives and many hydrophobic materials also possess disadvantages similar to those of low-surface-energy SAM chemistries, i.e., the molded material cannot be selectively bound and unbound during processing. The additives may also be quite expensive, as they tend to be proprietary formulations, or may require unique and complicated chemical synthesis to build precursor molecules. Furthermore, because the materials are UV-curable, their use is generally limited by the optical properties of the substrate. Use of transparent substrates may cause unintended release during micro/nanoprocessing, since materials for patterning are generally also sensitive to UV radiation. The additives are also typically designed to be cured by UV exposure, and are therefore applied permanently and generally are not removable without fouling the surface of the molded part. Thus, surfaces to which UV-curable formulations are applied typically cannot achieve switchable and reversible adhesion.